Solubility of salts Common-ion effect

Since the tie lines are not parallel, the point at which the two conjugate solutions have the same composition does not lie at the top of the binodal curve but off to one side at the point /c, the plait point. If the system has the composition d and acetic acid is added, the composition will move along dk just below k the two layers will both be present in comparable amounts at k the boundary between the two solutions vanishes as the system becomes homogeneous. Compare this behavior with that at c where only a trace of one of the conjugate layers remained. 

If the temperature is increased, the shape and extent of the two-phase region alters. A typical example for a system in which increase in temperature increases the mutual solubility is shown in Fig. 15.24. If temperature were plotted as a third coordinate, the two-phase region would be a loaf-shaped region. In the figure, P is the consolute temperature for the two-component system A-B. The line PQ connects the plait points at the various temperatures. 

If two of the pairs A-B and B-C are partially miscible, the situation becomes more complex. Two binodal curves can appear as in Fig. 15.25(a). At lower temperatures, the two binodal curves in Fig. 15.25(a) may overlap. If they do so in such a way that the plait points join one another, then the two-phase region becomes a band, as in Fig. 15.25(b). If the binodal curves do not join at the plait points, the resultant diagram has the form shown in Fig. 15.25(c). Points within the triangle abc represent states of the system in which three liquid layers having compositions a, b, and c coexist. Such a system is iso-thermally invariant. 

Unsaturated solution NH4CI + saturated solution (NH4)2S04 + saturated solution NH4CI + (NH4)2S04 a saturated solution c 

---